The basic function of a Thermogravimetric Analyzer (TGA) is to measure the amount and rate of weight change of a material, either as a function of increasing temperature, or isothermally as a function of time. During heating of a sample in a TGA, the sample may undergo changes that liberate gases. The TGA can interface with other analytical instruments (such as a gas chromatograph or mass spectrometer) for additional characterization of the evolved gases.
The basic purpose of a mass spectrometer (MS) is to convert a sample into products that are indicative of the original molecule. Vaporized sample molecules from a variety of possible inletting systems (typically a gas chromatograph or direct inlet probe) enter the source of the MS and interact with a beam of electrons to form a variety of products, including positive ions. This ion beam from the source is separated according to the respective masses of the ions by a variety of techniques, the two most common of which are magnetic deflection and quadropole filter. The masses and their relative abundances are displayed in a mass spectrum.
The mass spectra produced from individual compounds are a qualitative identification tool, and can be combined with other physical data for a compound to quantitatively characterize it.
Traditionally, mass spectrometers (MS) have also been used to act as detectors for gas chromatographs (GC). Moreover, the interface of a GC to an MS allows for the transfer of volatile species from a pressurized GC capillary column to an evacuated MS analysis chamber. Such an interface typically employs a transfer line which internally contains a splitting area wherein the pressure differential is equalized. In such a configuration, the ends of two capillary columns are brought into close proximity of each other, and samples from the pressurized GC column are swept by the inert gas flow into an evacuated capillary feeding into the MS. With a proper design, a MS can alternatively be used as a detector for a thermogravimetric analyzer (TGA). A GC-MS interface cannot function in a TGA-MS configuration for two reasons: 1) there is no capillary action from the TGA to deposit effluents in close proximity of the capillary column, and 2) there is no "head pressure" as exists with a GC to push concentrated effluents into the internal splitting area. As a result, insufficient quantities of volatiles make it into the MS.
Unlike the GC-MS configuration, wherein effluents remain under pressure, a TGA-MS combination allows for a sample to be outgassed at ambient pressure. This makes the TGA-MS combination particularly useful for understanding the behavior of compounds during conventional thermal processing. Additionally, it also allows for the identification of compounds that evolve during different stages of decomposition. These techniques have been successfully used in studies that range from thermal degradation of polymers to the analysis of sample contaminants.
The coupling of a MS to a TGA is particularly useful for understanding the real time behavior of the compounds at high temperatures. First, the detection in the MS occurs simultaneously as thermal reactions take place in the TGA. This allows for the identification of species that evolve at different temperatures and their correlation to the several weight loss regions. Secondly, the sample undergoing thermal decomposition in the TGA takes place at atmospheric pressure. This is a departure from conventional MS systems, wherein samples are typically introduced into the instrument via a solids introducing probe. Hence, the sample is vaporized in the vacuum. In contradistinction with the TGA used as the sample introduction system, vaporization of the sample occurs outside the MS at atmospheric pressure prior to its being drawn into the MS where ionization in vacuum takes place.
There are two major obstacles to overcome in interfacing a MS to a TGA. The first is the large pressure differential between the operating states of the two instruments. The low pressure (ranging from 10E-5 to 10E-8 torr) required for ionization and mass analysis in the MS is provided by a vacuum system, typically a turbomolecular or diffusion pumping system. The TGA, on the other hand, operates at atmospheric pressure. Any interface which couples the two systems must somehow compensate for this severe pressure differential.
The second obstacle to interfacing a MS to a TGA is the large mismatch in sensitivity between the TGA and the MS (MS being more sensitive to changes in a material by multiple orders of magnitude). An interface between the two techniques must somehow be able to reduce or dilute overly large amounts of volatile materials to avoid saturation of the MS.
FIG. 1 shows a prior art arrangement of a TGA linked to a MS via an interface, i.e., a capillary column of uniform diameter. The three essential components are shown: a thermobalance 1, shown as a DuPont Instruments 951 (TGA; a mass spectrometer 2, shown as a quadruple analyzer with accompanying RF generator 11; and the capillary interface 3 which couples the two devices. Operation of the furnace and the balance assemblies of the TGA (i.e., control of the sample temperature and measured sample weight) are controlled by a personal computer (PC) 12 with module interface. Volatiles from the sample heated in the TGA are pulled through the capillary via a pressure differential (i.e., reduced pressure at the MS end of the interface) and are subsequently leaked into the MS. In the MS, the incoming gas molecules are ionized and the ions (some of which have undergone fragmentation) are sorted in the quadruple analyzer. This sorting of ions is accomplished by changing the RF field between the poles of the analyzer, filtering the ions according to their mass-to-charge ratios. The entire operation of the MS (including RF generator) is controlled by a second PC 13.
E. L. Charsley, et al. in an article entitled "Thermogravimetry-mass spectrometry using a simple capillary interface", published in American Laboratory, January 1990, describes a TGA-MS system shown in FIG. 1, that comprises a thermobalance 1, a mass spectrometer 2, and an interface coupling the two apparatus. Linking the TGA equipment to a MS presents a problem that each of the two apparatus operate at different pressures. Whereas the TGA normally operates in an atmospheric pressure environment, the MS must operate below 10.sup.-5 mbar. To reduce the pressure of the carrier and of the evolved gas prior to analysis, a flexible and inert fused silica-lined stainless steel capillary 3 is used, having an inner diameter approximately 0.3 mm, and a length ranging from 6" to 4'. The capillary has one end positioned in the thermobalance in close proximity of the sample 4, and the other 5 is connected to the second stage of a rotary pump (not shown) linked with the MS. In this manner, the gas flow to the MS is drastically reduced. The alumina probe 6 positioning the capillary 3 close to the sample 7, e.g., a polymer, has a separate resistance heater (not shown), preferably connected in series to a stainless steel sheath 8. Both ends of the capillary column are respectively secured by a Teflon connector block 9 and a seal 10.
A significant drawback of the interface linking a TGA to a MS as described in FIG. 1 lies in the difficulty of adjusting the concentration of volatiles that are introduced in the MS, a prime requirement in view of the inherent mismatch in sensitivity between the TGA and the MS.